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Beta-lactam antibiotic (natural penicillin) Pregnancy: Benzylpenicillin sodium has been taken by a large number of pregnant women and women of childbearing age without an increase in malformations or other direct or indirect harmful effects on the foetus. Breast-feeding: it is not known whether it is excreted into human breast milk, but it is actively transported from blood to milk in animals and trace amounts of other penicillins have been detected in human milk.

Benzylpenicillin (Penicillin G)

Brand names: Crystapen

Used in: Meningitis & Encephalitis

Benzylpenicillin (penicillin G) is a narrow-spectrum beta-lactam antibiotic given parenterally to treat serious infections such as meningococcal disease, streptococcal infections and as part of empirical therapy for suspected bacterial meningitis.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 600 mg to 3,600 mg (1 to 6 mega units) daily, divided into 4 to 6 doses, depending on the indication
Route: Intravenous or intramuscular injection; high doses by intravenous injection or infusion
Frequency: In 4 to 6 divided doses daily
Max: Up to 14.4 g/day (24 mega units) in divided doses may be given in serious infections such as adult meningitis by the intravenous route; doses up to 43.2 g (72 mega units) per day may be necessary for rapidly spreading gas gangrene
Bacterial endocarditis: 7.2 to 12 g (12 to 20 mega units) or more daily in divided doses by the intravenous route, often by infusion. Meningococcal disease, adults and children over 12 years: 2.4 g every 4 hours. SUSPECTED meningococcal disease in the community — general practitioners should give a single dose before transfer to hospital: adults and children over 10 years 1,200 mg IV (or IM); children 1-9 years 600 mg IV (or IM); children under 1 year 300 mg IV (or IM). Prevention of Group B Streptococcal disease of the newborn: 3 g (5 mega units) loading dose to the mother, then 1.5 g (2.5 mega units) every 4 hours until delivery. ADMINISTRATION RATE: intravenous doses in excess of 1.2 g (2 mega units) must be given slowly, taking at least one minute for each 300 mg (0.5 mega unit), to avoid CNS irritation and/or electrolyte imbalance. Reconstitution: 1200 mg (2 mega units) in at least 8 mL for IV injection, or at least 20 mL for IV infusion, of sodium chloride injection BP or water for injections BP; do not reconstitute in sodium-containing liquids in patients with renal failure and/or heart failure (risk of sodium overload). High dosage may cause hypernatraemia and hypokalaemia unless the sodium content is taken into account. Elderly: elimination may be delayed and dose reduction may be necessary. Use only freshly prepared solutions.

Paediatric dose

Dose: 100 mg/kg
Route: Intravenous or intramuscular
Frequency: Per day, in 4 divided doses (children aged 1 month to 12 years)
Max: Not exceeding 4 g/day
SPC standard paediatric dosing: children 1 month to 12 years 100 mg/kg/day in 4 divided doses, not exceeding 4 g/day; infants 1-4 weeks 75 mg/kg/day in 3 divided doses; newborn infants 50 mg/kg/day in 2 divided doses. MENINGOCOCCAL DISEASE: children 1 month to 12 years 180-300 mg/kg/day in 4-6 divided doses, not exceeding 12 g/day; infants 1-4 weeks 150 mg/kg/day in 3 divided doses; newborn infants 100 mg/kg/day in 2 divided doses; children over 12 years 2.4 g every 4 hours. SUSPECTED meningococcal disease pre-hospital single dose: children over 10 years 1,200 mg IV/IM; 1-9 years 600 mg IV/IM; under 1 year 300 mg IV/IM. Premature babies and neonates: dosing should not be more frequent than every 8 or 12 hours because renal clearance is reduced and the half-life may be as long as 3 hours; intravenous treatment is preferred to intramuscular because infants may develop severe local reactions. Verify all paediatric dosing against a children's formulary before administration.

Dose adjustments

Renal

For doses of 0.6-1.2 g (1-2 mega units) the dosing interval should be no more frequent than every 8-10 hours. For high doses (e.g. 14.4 g/24 mega units for serious infections such as meningitis) adjust per the SPC schedule by creatinine clearance (mL/min): 125 — 1.2 or 1.8 g (2 or 3 mega units) every 2 or 3 hours; 60 — 1.2 g (2 mega units) every 4 hours; 40 — 0.9 g (1.5 mega units) every 4 hours; 20 — 0.6 g (1 mega unit) every 4 hours; 10 — 0.6 g (1 mega unit) every 6 hours; nil — 0.3 or 0.6 g (0.5 or 1 mega unit) every 6 or 8 hours. Reduce further to 300 mg (0.5 mega units) 8-hourly if advanced liver disease is associated with severe renal failure. If haemodialysis is required, give an additional 300 mg (0.5 mega units) 6-hourly during the procedure. In impaired renal function large doses can cause cerebral irritation, convulsions and coma.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Paediatric weight-based calculator

SPC standard paediatric dosing: children 1 month to 12 years 100 mg/kg/day in 4 divided doses, not exceeding 4 g/day; infants 1-4 weeks 75 mg/kg/day in 3 divided doses; newborn infants 50 mg/kg/day in 2 divided doses. MENINGOCOCCAL DISEASE: children 1 month to 12 years 180-300 mg/kg/day in 4-6 divided doses, not exceeding 12 g/day; infants 1-4 weeks 150 mg/kg/day in 3 divided doses; newborn infants 100 mg/kg/day in 2 divided doses; children over 12 years 2.4 g every 4 hours. SUSPECTED meningococcal disease pre-hospital single dose: children over 10 years 1,200 mg IV/IM; 1-9 years 600 mg IV/IM; under 1 year 300 mg IV/IM. Premature babies and neonates: dosing should not be more frequent than every 8 or 12 hours because renal clearance is reduced and the half-life may be as long as 3 hours; intravenous treatment is preferred to intramuscular because infants may develop severe local reactions. Verify all paediatric dosing against a children's formulary before administration.

Verify in a children's formulary

Contraindications

  • Allergy to penicillins
  • Hypersensitivity to any ingredient of the preparation
  • Cross-allergy to other beta-lactams such as cephalosporins should be taken into account; not to be used at all in patients with a history of severe hypersensitivity reactions to other beta-lactams

Side effects

  • Hypersensitivity in the form of rashes (all types), fever and serum sickness (common, 1-10%); anaphylactic reactions (rare, <0.05%); angioedema (frequency not known)
  • Jarisch-Herxheimer reaction in patients treated for syphilis or neurosyphilis (very common)
  • Granulocytopenia (neutropenia), agranulocytosis, leucopenia with prolonged high doses (rare); anaemia, thrombocytopenia (not known); Clostridium difficile diarrhoea
  • CNS toxicity including convulsions with massive doses over 60 g per day and in severe renal impairment (rare); metabolic encephalopathy (not known)
  • Interstitial nephritis after intravenous doses of more than 12 g per day (rare); severe cutaneous adverse reactions (SJS, TEN, DRESS, AGEP) reported with beta-lactams

Interactions

  • Note: section 4.5 content was truncated in this source bundle and no interactions were captured — clinician to verify against the SPC

Clinical monograph

How it works

It inhibits bacterial cell wall synthesis by binding penicillin-binding proteins, disrupting peptidoglycan cross-linking and leading to bacterial cell lysis.

Prescribing in practice

  • It is contraindicated in patients with a history of penicillin or other beta-lactam hypersensitivity, and anaphylaxis can occur; take a careful allergy history.
  • It is given by injection or infusion as it is poorly absorbed orally, and the dose is reduced in significant renal impairment.
  • It is recommended for immediate administration in suspected meningococcal disease in the community where transfer to hospital is delayed, per current prescribing references.

Monitoring

Monitor for hypersensitivity reactions and clinical response, with attention to renal function when high parenteral doses are used.

Counselling the patient

  • Tell the team if you have ever had a reaction to penicillin or other antibiotics.
  • This antibiotic is given by injection or a drip into a vein.
  • Report any rash, swelling or breathing difficulty immediately.

Evidence & guidelines

Benzylpenicillin is a long-established antibiotic and is recommended in UK guidance for pre-hospital treatment of suspected meningococcal disease.

Reference: NICE NG240; PHE Meningococcal Guidelines; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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Featured in these MRCEM clinical pathways

Benzylpenicillin (Penicillin G) is a core drug in the following exam-focused workups on our sister siteReviseMRCEM.

MRCEM Primary / Intermediate / OSCE candidates: each pathway includes exam-style questions, RCEM/NICE citations, and FAQ summaries.